In digital watermarking, an imperceptible signal referred as a watermark is embedded into multimedia data for various purposes such as copyright protection, fingerprinting, authentication, etc. The embedding of the watermark usually introduces irreversible distortion, although it may be quite small, in the original data. For applications where the availability of original data is essential, irreversible degradation of the original data is not acceptable, and incurred distortions need to be removed. Examples of such applications include multimedia archives, military image processing, and medical image processing for electronic patient records (EPRs) [1]. In multimedia archives, a content provider may not want the original content to be distorted even though the distortion is imperceptible to most users, and it may be too costly in terms of storage space to store both the original and the watermarked versions. In military image processing, images are gathered at a very high cost and are usually subjected to further processing steps such as extreme magnification. Any distortion may hinder accurate analysis of the image. In medical image processing, any modification to the original image may affect a doctor's diagnosis and lead to legal problems.
Any complications that can occur when using a conventional watermarking scheme in the applications listed above can be resolved by using the reversible (lossless, invertible, erasable, etc.) watermarking scheme. Although the embedding distortion is inevitable even in reversible watermarking, it can be removed, and the original data can be reconstructed from the watermarked version. Another advantage of the reversible watermarking is that the access to the original content can be controlled. In a conventional watermarking scenario, no one has access to the original content since the distortion due to the embedding of the watermark is not erasable. When the watermark is embedded in a reversible manner, an authorized person can access the original content by erasing the watermark, while the watermarked content is available to everyone.
When the original content can be recovered from the watermarked content, the watermarking scheme is said to have the reversibility (invertibility) property. Note that the reversibility property can also be obtained using standard (cryptographic) scrambling algorithms. However, the cryptographic approach completely obliterates any semantic understanding, which is not the case in reversible watermarking.
Recently, several reversible watermarking schemes have been proposed [2]-[17]. The concept of a reversible watermark was first introduced by Mintzer et al. [2]. The watermark that they embedded into an image was completely visible but could be removed since it was embedded in a reversible manner. Fridrich et al. extracted a vector which represented specific characteristics of pixel groups, compressed it without any loss, and embedded the watermark bits by appending it to the compressed vector [3]. Tian applied integer Haar wavelet transform to an image and embedded the watermark into high-frequency coefficients by difference expansion (DE) [4]. Alattar extended Tian's scheme and applied the DE to triplets [5] and quads [6] of adjacent pixels for reversible embedding. He also proposed a reversible watermarking scheme using the DE of a generalized integer transform [7]. Veen et al. applied the companding technique to reversibly embed a large amount of data into an audio signal [8]. Leest et al. applied Veen's method to image watermarking [9]. Vleeschouwer et al. proposed a lossless watermarking based on circular interpretation of bijective transformations [10]. Celik et al. generalized a well-known least-significant bit (LSB) substitution technique and achieved high capacity by using a prediction-based conditional entropy coder [11] [12]. Yang et al. proposed a reversible watermarking scheme based on integer DCT transform [13]. Xuan et al. reversibly embedded the watermark bits into the middle bit-plane of the middle and high frequency integer wavelet coefficients [14]. Kalker and Willems derived a theoretical bound on the embedding capacity for reversible data hiding [15]-[17].
In conventional reversible watermarking schemes, however, high embedding capacity without degrading the image quality can not be sufficiently achieved, so more advanced reversible watermarking schemes are being highly needed.